This invention relates to a device which holds a cylindrical end-sensitive sensor of the variety in commerial use for measuring pressure or other physiological functions. The device securely connects the sensor to the skull in such relationship to the brain as to provide for optimal electronic moitioring of brain surface pressure.
Intracranial pressure has been measured by catheters connecting the fluid within the cerebral ventricles to a variety of electronic and non-electronic pressure monitors, and brain tissue pressure has been measured by placing small transducers within the tissues of the brain. In order to avoid penetration of the brain by the monitoring device, brain surface pressure has been measured by hydraulically linking a fluid pool trapped beneath the skull with a pressure transducer located outside the skull. With this method leakage of coupling fluid or obstruction of the coupling channel by tissue often results in inaccurate measurements. To avoid the problems with fluid coupling, a variety of intracranial pressure measuring devices have been designed which allow the pressure sensor to be positioned in the space between the brain and the skull, with the wire leading from the sensor to the electronic reading device exiting the skull through a drilled insertion hole beside the sensor. Orientation of the pressure sensitive surface of such devices is uncertain. After these devices have been implanted, a second surgical operation is required to inspect, calibrate, or replace the sensor.
The use of end-sensitive pressure sensors applied to the membrane which covers the surface of the brain, the dura mater, and positioned tangential to the surface of the brain has been described. Certain devices used for such measurement have secured the pressure sensitive surface of the transducer the the skull by screw threads in the bone, by expansion of a pressfit ring surrounding the transducer to snugly fit the hole in the skull. Neither arrangement provides for non-surgical removal of the transducer. Additionally, none assures correct placement of the pressure sensitive surface at the critical depth beneath the skull, nor assures that, in the event of a tilted hole the pressure sensitive surface will be applied in a tangential manner to the surface of the brain. Furthermore, the screw-thread device is associated with the risk of bone chips being driven ahead of the male screw. In addition to the potential of chip penetration, displacement of the surface of the brain away from the pressure sensitive surface of the transducer by the chips results in an inaccurate pressure reading.
Certain sensor holders or anchoring devices are supplied in the form of cylinders which fit into a hole in the skull. An outer flange on the cylinder larger than the hole in the skull limits the depth of pentration of the cylinder into the hole in th skull. Such devices have been described in U.S. Pat. Nos. 4,265,252 and 4,705,499. These devices do not allow for the irregularity in the contour of the inner or outer tables of the skull, or for the normal variation in thickness of the skull in different individuals, and therefore do not assure that contact between the sensitive surface of the sensor and the dura mater is proper for accurate pressure measurement.
Other sensor anchoring devices incorporating lugs or cams on the tip of the anchoring device which engage the inner table of the skull and prevent withdrawal of the sensor tip through the hole have been described in U.S Pat. Nos. 4,062,354; 4,494,411; and 4,805,634. All of these devices require that the hole in the skull by precisely and uniquely tailored to accommodate the sensor anchoring device. This is a direct result of their inability to self-adjust to the varying conditions of skull thickness, hole configuration and simple surgical technique.
In the anchoring device of U.S. Pat. No. 4,062,354 side enlargements to the hole are required to permit passage of the lugs through the hole. The holding bracket is rotated after insertion to bring the lugs against the inner table of the skull. An internally threaded ring is then screwed onto outer threads on the anchoring device and tightened against the outer table of the skull, the skull thereby being clamped between the lugs and the ring.
In the anchoring devices of U.S. Pat. Nos. 4,494,411 and 4,805,634 a cylinder having spring ribs with beads on their ends is inserted through the hole. The ribs are forced outward by inserting a second cylinder or a cylindrical sensor into the anchoring device. Both of these anchoring devices require that a hole with two different diameters, a larger outer hole abruptly reduced to a smaller diameter inner hole be drilled through the skull. The precise depth of the sensor tip is determined by the distance between the inner table of the skull and the bottom of the outermost hole.
In summary, prior art for measurement of intracranial pressure requires penetration of the sensor into the brain; or measurement of pressure in a pocket of trapped fluid which may dissipate; or placement of a sensor between the skull and the brain, with the risk of improper orientation of the sensor, or use of a sensor anchored to the skull with a device which may drive bone chips ahead of it toward the brain when it is applied, which may not correctly orient the sensor, or which requires absolute technical precision and custom instruments for surgical placement. Furthermore, the anchoring devices of proir art require a second surgical procedure to remove the sensor for calibration or replacement. The method for measurement of intracranial pressure described herein and the sensor anchoring device of this application are designed to avoid the problems enumerated with the aforementioned methods of intracranial pressure measurement and anchoring devices.